Modern aircraft need to convert the variable frequency power provided by engine rotation into constant frequency electrical power. One method of doing this is through the use of a solid state variable speed constant frequency (VSCF) system in conjunction with a generator. Because this equipment generates heat during use, adequate cooling must be provided to prevent an overtemperature condition from developing. Prolonged durations at high temperatures could impact the reliability or destroy the electronic equipment. An overtemperature condition could occur because of a sudden increase in power dissipated by the electronic equipment. Alternatively, an overtemperature condition could also occur when there is no increase in power dissipation but there is loss of cooling.
Thus, there are two problems which cooling system designers currently face. The first is how to prevent equipment from experiencing prolonged durations at high temperatures in the event of a sudden increase in power dissipation by that equipment. There are many situations which can cause this sudden increase to occur. For example, such a conversion system will normally operate at only one-half to two-thirds capacity. At times, it may be necessary to run the system at full capacity which will result in a significant increase in total heat generated by the system. In a second example, such a conversion system may serve only as a back-up and will therefore only require cooling during emergency situations. The cooling system must be designed so that worst case needs may be met, otherwise the conversion system will sustain damage. It is wasteful, however, to provide at all times cooling sufficient for worst case conditions. An aircraft engine must supply bleed air to the cooling system; therefore, requiring the engine to supply more air than is necessary having a negative impact on engine operating efficiency.
The second problem is how to provide redundant cooling to essential systems which will be destroyed almost immediately in the event of no cooling. Loss of electrical power, for example, would prove catastrophic for an airplane with electrical controls. Primary cooling systems may fail for any number of reasons, such as lost coolant pressure or blocked cooling channels.
Best, in U.S. Pat. No. 2,953,078, recognizes the need to provide adequate cooling to various compartments of an aircraft. The Best invention relates to an improved system for distribution of air to the cabin and various compartments of an aircraft which require air conditioning. The Best invention contemplates the use of ram air to supplement and/or replace an engine compressor supply for cooling the external compartments in an integrated distribution system. The cold air supply is distributed by a network of conduit and valves. The Best invention, however, neither provides redundant cooling for essential hardware nor does it accommodate hardware whose peak cooling needs substantially exceed their nominal cooling needs.
With reference to FIG. 1 one prior art improvement on the Best concept utilizes an auxiliary coolant channel or conduit 24" which is linked to a main channel 14 before it reaches the heat generating apparatus 44. In this scheme, a temperature sensor (not illustrated) monitors the temperature of the apparatus. A controller responsive to this sensor activates an auxiliary fan 28 in the auxiliary channel in the event of an overtemperature condition of the apparatus. The auxiliary fan 28 creates a pressure in the auxiliary channel greater than that in the main channel, causing check valves 18 and 32 responsive to air pressure to switch the coolant source coupled to the electronics 44 from the main channel to the auxiliary channel. In doing this, the check valves 18 and 32 prevent the air mass flow in the auxiliary channel 24 from reaching paths which need no additional cooling. This is an improvement over the Best invention in that the cooling system is responsive to an overtemperature condition, whether it be caused by main cooling system failure or by increased thermal energy production. However, for maximum efficiency, the apparatus should receive all of the coolant from the auxiliary channel in addition to a predetermined share of coolant from the main channel. This scheme allows for only one channel to be used at a time, because it does not provide for an auxiliary channel that is separate and free of fluid communication with the main channel. Therefore, this scheme results in an auxiliary cooling system which is larger than necessary.
The operation of the prior art of FIG. 1 is described as follows. During normal operation, heat generating apparatus 44 receives a predetermined share of the coolant supplied by a main coolant channel 14 and it receives no coolant from an auxiliary conduit 30. During an overtemperature condition; cooling controller 40, which is responsive to the temperature of heat generating apparatus 44, commands auxiliary fan 28 to turn on. This creates an air pressure in conduit 30 which is greater than that in conduit 24 and conduit 22, forcing check valve 32 open and check valve 18 closed. This effectively switches the source of coolant for apparatus 44 from main conduit 14 to auxiliary conduit 30. Check valve 18 prevents coolant in auxiliary conduit 30 from flowing into main conduit 16 and prevents coolant in main conduit 16 from reaching apparatus 44. This design does not provide the heat generating apparatus 44 with two separate channels free of fluid communication or mixing, thereby preventing the heat generating apparatus from receiving its predetermined share of coolant from the main channel 14. This coolant will therefore be wasted, since compartments (not shown) which are connected to conduit 16 need no additional coolant. If this coolant could reach apparatus 44, then an auxiliary cooling system could be designed smaller. The present invention accomplishes this by providing separate channels, free of fluid communication or mixing, from the coolant source to the heat generating apparatus for the constant and temporary sources.